Send token on error message.

When en error occured on a CoAP request, the response does not contain the Token. It seems this is a bug.

See [CoAP specification](https://datatracker.ietf.org/doc/draft-ietf-core-coap/?include_text=1) :

_"**2.2.  Request/Response Model** ..._

       Client              Server       Client              Server
          |                  |             |                  |
          |   CON [0xbc90]   |             |   CON [0xbc91]   |
          | GET /temperature |             | GET /temperature |
          |   (Token 0x71)   |             |   (Token 0x72)   |
          +----------------->|             +----------------->|
          |                  |             |                  |
          |   ACK [0xbc90]   |             |   ACK [0xbc91]   |
          |   2.05 Content   |             |  4.04 Not Found  |
          |   (Token 0x71)   |             |   (Token 0x72)   |
          |     "22.5 C"     |             |   "Not found"    |
          |<-----------------+             |<-----------------+
          |                  |             |                  |

I propose this patch which change a very small amount of code, but maybe this is not the best way.
Probably the  `lwm2m_handle_packet` function should be refactored because it begins to be long, but I do not feel to change it :), I do not manage this part of code enough.